The Evolution of Massive Stars. I. Red Supergiants in the Magellanic Clouds

By Philip Massey, Kirsty Olsen. 2003

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We investigate the red supergiant (RSG) content of the SMC and LMC using multiobject spectroscopy on a sample of red stars previously identified by BVR CCD photometry. We obtained high-accuracy (<1 km s-1) radial velocities for 118 red stars seen toward the SMC and 167 red stars seen toward the LMC, confirming most of these (89% and 95%, respectively) as red supergiants. Spectral types were also determined for most of these RSGs. We find that the distribution of spectral types is skewed toward earlier type at lower metallicities: the average (median) spectral type is K5–K7 I in the SMC, M1 I in the LMC, and M2 I in the Milky Way. Our examination of the Kurucz ATLAS9 model atmospheres suggests that the effect that metallicity has on the appearance on the TiO lines is probably sufficient to account for this effect, and we argue that RSGs in the Magellanic Clouds are 100 K (LMC) and 300 K (SMC) cooler than Galactic stars of the same spectral types. The colors of the Kurucz models are not consistent with this interpretation for the SMC, although other models (e.g., Bessell et al.) show good agreement. A finer grid of higher resolution synthetic spectra appropriate to cool supergiants is needed to better determine the effective temperature scale. We compare the distribution of RSGs in the H-R diagram to that of various stellar evolutionary models; we find that none of the models produce RSGs as cool and luminous as what is actually observed. This result is much larger than any uncertainty in the effective temperature scale. We note that, were we to simply adopt the uncorrected Galactic effective scale for RSGs and apply this to our sample, then the SMC's RSGs would be underluminous compared with the LMC's, contrary to what we expect from stellar evolution considerations. In all of our H-R diagrams, however, there is an elegant sequence of decreasing effective temperatures with increasing luminosities; explaining this will be an important test of future stellar evolutionary models. Finally, we compute the blue-to-red supergiant ratio in the SMC and LMC, finding that the values are indistinguishable (∼15) for the two Clouds. We emphasize that "observed" B/R values must be carefully determined if a comparison with that predicted by stellar models is to be meaningful. The nonrotation Geneva models overestimate the number of blue to red supergiants for the SMC, but underestimate it for the LMC; however, given the inability to produce high-luminosity RSGs in the models that match what is observed in the H-R diagram, such a disagreement is not surprising.

Subfields: Astronomy and Astrophysics, Education, Instrumentation

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Citations in the corpus

Variability in red supergiant stars: pulsations, long secondary periods and convection noise (2006, background): “In particular, stellar evolutionary models seemed unable to produce RSGs that are as cool or as luminous as observed (Massey 2003, Massey & Olsen 2003).”
Production of dust by massive stars at high redshift (2011, background): “Stars more massive than 25 M⊙ evolve into WR stars and likely explode as Ib or Ic SNe (Massey and Olsen, 2003; Crowther, 2007).”
SPITZER SAGE INFRARED PHOTOMETRY OF MASSIVE STARS IN THE LARGE MAGELLANIC CLOUD (2009, background): “For comparison, we also included 147 red supergiants from Massey & Olsen (2003), some of which were originally classified by Humphreys (1979).”
A Massive Cluster of Red Supergiants at the Base of the Scutum-Crux Arm (2007, background): “Median spectral types of K5, M1, DAVIES ET AL.786 Vol. 671 and M2 have been found from studies of the SMC, LMC, and the Galaxy, respectively (Humphreys 1979; Elias et al. 1985; Massey & Olsen 2003).”
An Empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich asymptotic giant branch stars (2005, background): “Of these the vast majority, stars with MZAMS ∼8–30 M⊙ pass through a red supergiant (RSG) stage, when they both are very luminous, LRSG ∼ 105 L⊙, and become cool, Teff,RSG ∼ 3500–4000 K (Massey & Olsen 2003).”
MESA ISOCHRONES AND STELLAR TRACKS (MIST). I. SOLAR-SCALED MODELS (2016, methodology): “In Figure 36, we compare the MIST evolutionary tracks in black and the sample of observed RSGs from Massey et al. (2009).”
Pulsational Pair-instability Supernovae (2016, background): “…however, with observations showing that stars above about 35 M , the “Humphreys-Davidson limit” (Humphreys & Davidson 1979), do not spend a significant part of their lifetime as RSGs, even in the SMC (Massey & Olsen 2003; Levesque et al. 2007) where the metallicity is about one-seventh solar.”
THIRD-EPOCH MAGELLANIC CLOUD PROPER MOTIONS. II. THE LARGE MAGELLANIC CLOUD ROTATION FIELD IN THREE DIMENSIONS (2013, methodology): “For our young sample, we combined the RSG velocities of Prevot et al. (1985), Massey & Olsen (2003), and O11 (adopting the classification from their figure 1).”
Evolutionary stellar population synthesis at high spectral resolution: optical wavelengths (2005, background): “More recently, Massey & Olsen (2003) have found that the Padova and Geneva models are not able to produce RSGs as cool and luminous as what is actually observed.”
Discovery of an Extraordinarily Massive Cluster of Red Supergiants (2006, background): “McGregor & Hyland (1984) and Massey & Olsen (2003) discuss RSGs in open clusters in the SMC, LMC, and Galaxy.”
The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought (2005, unknown): “The lower abundance of TiO may by itself lead to a lower Teff for stars of the same spectral subtype, as suggested by Massey & Olsen (2003).”
The Effect of Massive Binaries on Stellar Populations and Supernova Progenitors (2007, methodology): “Our luminosity limit reduces contamination from AGB stars (Massey & Olsen 2003) in the RSG sample.”
On the Progenitor of SN 2005gl and the Nature of Type IIn Supernovae (2006, background): “…they are generally rare in more normal galaxies (with but a single SSC candidate, Westerlund 1, observed in our Galaxy) and are expected to be even less frequent in 7 Massey & Olsen (2003) have recently identified very luminous red supergiants in the LMCwith bolometric luminositiesM > 10 mag.”
Scaled solar tracks and isochrones in a large region of the Z-Y plane. II. From 2.5 to 20 solar masses (2009, methodology): “…of the extra mixing should decrease with increas- ing stellar mass. Clouds were too cool and luminous compared with evolution- ary tracks (Massey 2003; Massey & Olsen 2003) until when Levesque et al. (2005, 2006) derived a new effective tempera- ture scale and new bolometric corrections,…”
The Effective Temperatures and Physical Properties of Magellanic Cloud Red Supergiants: The Effects of Metallicity (2006, methodology): “The preliminary spectral types assigned by Massey & Olsen (2003) were used to ensure that a good range of spectral subtypes were observed in each MC.”
Binary Population and Spectral Synthesis Version 2.1: Construction, Observational Verification, and New Results (2017, methodology): “For all these number and stellar type number ratios, we only include stars with log (L/L ) > 4.9, since Massey & Olsen (2003) used this as a luminosity cut-off to make sure that AGB stars are not included in their sample (although we note that a small number of our AGB models do indeed satisfy this…”